1. Field of the Invention
The present invention relates to a picture transmission system based upon a frequency multiplex technique or a time division multiplex technique, applicable to on-demand video systems, monitoring camera systems, and others, which transmission system is capable of effective allocation of transmission resources and of switching operation producing less disorder or disturbance of pictures.
2. Description of the Prior Art
Recently, a transmission system for multiplexing video information or audio information to transmit them through a transmission medium has been adopted in fields such as cable television systems and monitoring camera systems which need to transmit a plurality of video information. A description will be made hereinbelow of a prior picture transmission system. As exemplified by Japanese Unexamined Patent Publication No. 5-7330, there has been known a transmission system using a transmission medium allowing the multiplexing of a plurality of video information. Although in this publication a picture transmission system is constructed as a monitor camera system, since even in a picture transmission system of a cable television the picture inputting means (cameras) can be considered to be centered at a head end portion without scattered, in addition to the TV camera a video player or the like can also be taken as the picture inputting means. Accordingly, the cable television system can be regarded as being a prior art.
FIG. 30 shows an arrangement of a prior transmission system.
This prior transmission system is composed of picture inputting means (cameras) 3201-1 to 3201-n, a transmission medium 3202 for multiplexing and transmitting video information inputted through the picture inputting means, picture output means (monitors) 3203-1 to 3203-m for displaying the transmitted video information, branching means 3204 fitted in the transmission medium 3202 to join and distribute the multiplexed information, picture fixed transmission units 3205-1 to 3205-n for fixedly outputting the video information from the picture inputting means to specific channels of the transmission medium 3202, and picture receiving units 3206-1 to 3206-m for selecting the video information in the specific channels from the video information in a plurality of channels in the transmission medium 3202 to output them to the picture outputting means 3203-1 to 3203-m.
A description will be taken hereinbelow of an operation of the picture transmission system thus constructed. The picture fixed transmission units 3205-1 to 3205-n modulate the video information, piked up by the respective picture inputting means (cameras) 3201-1 to 3201-n, with different fixed channel resources on the transmission medium 3202, with the modulated video information signals being introduced through the branching means 3204 into the transmission medium 3202. On the other hand, in the reception side the branching means 3204 distribute the video information signals on the transmission medium 3202, while the picture receiving units 3206-1 to 3206-m demodulate the video information signals through the use of the channel resources designated, so that the picture outputting means (monitors) 3203-1 to 3203-m display the demodulated video information. The picture picked up by the a given picture inputting means (camera) can appear on the picture outputting means (monitors) in such a manner that the operator changes the demodulation channels of the picture receiving units 3206-1 to 3206-m.
However, the prior picture transmission system creates the following problems. First, since the channel resources for the video information fed from the picture inputting (camera, video player) side are in a fixed condition, the number of picture inputting means to be installed is not be allowed to exceed the number of transmission medium channel resources. In addition, in practice the restriction to the channel resources usable occurs due to the significance and installed place of each of the picture inputting means (cameras), and the conventional arrangement involves this restriction in determining a different channel resource at every picture sending unit. Further, a special managing unit is necessary for the dynamic allocation of channel resources, and the trouble of the managing unit causes the difficulty of switching pictures. Still further, since difficulty is encountered to simultaneously carry out the picture switching operations in such a manner that a plurality of picture receiving units and a plurality of picture sending units are in the synchronized relation to each other, the picture temporarily falls into disorder at the picture switching.
Moreover, since the stability of a reception circuit in the picture receiving unit takes time in switching the channel resources for the picture reception, the disorder of the picture temporarily occurs. Further, when the picture receiving unit or the picture sending unit transmits a control signal to all the picture receiving units or picture sending units, the retransmission requests issued from the picture reception units or the picture sending units failing to receive the control signal gather to deteriorate the performance of the transmission medium. Still further, in the case of automatically performing the operations such as picture switching according to a given procedure, a special managing unit is required to manage that procedure, besides the switching processing and the transmission time go on increasing due to the use of the managing unit, which makes it difficult to gain a sufficient performance.
Furthermore, in the case of automatically executing the operation such as the picture switching with no use of the managing unit, difficulty is experienced to control its own automatic execution such as temporary stop and reexecution. In addition, in the case of automatically implementing the operation such as the picture switching with no use of the managing unit, since the scattered picture receiving units or picture sending units carry out the automatic execution, it is difficult to grasp the state of the execution. Further, in the case of the automatic execution of the operation such as the picture switching, there is a need to send switching requests, different in contents from each other, through the transmission medium to a plurality of picture receiving units or picture sending units, and therefore the actual switching operations therebetween can not concurrently take place, so that the picture switching in the plurality of picture receiving units take place in time-shifted states and becomes unnatural. Still further, in the case of the prior system, difficulty is encountered to transmit, through the transmission medium, control signals issued from control equipment (a camera, a video player, a different system or the like which outputs an message). Besides, because the picture receiving units or the picture sending units, which independently operate in parallel to each other, conduct various processes, it is difficult to capture the situation of the occurrence of errors.